This invention relates to digital oscilloscopes and more particularly to the display of waveforms acquired from a system under test.
Digital oscilloscopes are used in the art to acquire and display waveforms from a system under test. Single-shot oscilloscopes are used to examine nonrepetitive waveforms, while repetitive sampling oscilloscopes are used to examine repetitive waveforms from a system under test. Repetitive sampling oscilloscopes make several separate acquisitions of data which are combined to provide the user with a complete picture of the waveform at a chosen resolution.
One method of acquiring data consists of continuous sampling of the waveform under test, transforming the analog signals to digital signals, then storing the digital results in a defined circular memory. Once a user-defined trigger event occurs, samples continue to be acquired until the number is sufficient for display purposes, then the continuous sampling is halted. Hardware interpolators in a timebase circuit determine the time relationship between the trigger event and the acquired samples, and the acquired samples (one acquisition) are displayed relative to the trigger event. FIG. 1A shows a waveform under test. FIG. 1B shows a timing diagram for a sample clock. A trigger level is chosen by the user for the waveform under test as shown in FIG. 1A, a trigger event occurs in FIG. 1A, and is indicated in FIG. 1C. Assuming samples of the waveform under test are acquired at each rising edge of the sample clock, the acquired samples would be displayed by a digitizing oscilloscope as shown in FIG. 1D. The continuous sampling is resumed until the trigger event occurs again, and the process is repeated to obtain a second acquisition, which is also displayed. FIG. 2A shows the oscilloscope display of a waveform under test after one acquisition, while FIG. 2B shows the same display after two acquisitions. This process is repeated continuously, and the accumulation of acquisitions is displayed by the oscilloscope, as shown in FIG. 2C.
The display of a digital oscilloscope is usually a raster display which consists of rows (x-axis) and columns (y-axis) of discrete pixels. The accumulation of acquisitions are mapped onto the display, with time of the sample placed on the x-axis and the value of the sample on the y-axis. The user chooses the length of time to be displayed from a range of predetermined choices, and thereof determines the period of time each column represents. Only one sample value is represented in each column, so the occurrence of a new sample value during an acquisition in a particular column having an existing value causes the existing value to be erased and the new sample value displayed. Consequently, the resolution of the oscilloscope is limited to the particular time length chosen by the user. The maximum resolution of the oscilloscope is controlled by the maximum resolution of the hardware interpolators, assuming the interpolators have a higher resolution than the sample clock.
Often a user finds it advantageous to choose a time length which displays a relatively large period of time, thus permitting the user to observe the primary characteristics of the waveform. After observing the waveform at this resolution, the user often desires to observe a portion of the displayed waveform at a higher resolution, and make measurements of the waveform which are more accurate because of the higher resolution of the display. One example of a measurement is the 10-90% rise time. FIG. 3A shows a relatively low resolution display of a waveform under test. Because of the limited number of points which can be displayed on the rising edge, the 10-90% risetime measurement is not very accurate. FIG. 3B shows a higher resolution display (relative to FIG. 3A) of the same waveform. Measurements made on the waveform under test (based on the displayed points) will be more accurate, because the higher resolution of the display allows more rising edge points to be displayed.
In the prior art, two solutions are available. In an oscilloscope having a single timebase circuit for displaying a waveform, the oscilloscope must be reconfigured for a higher resolution by choosing a different time length from the predetermined choices, and then choosing the particular time period which the user desires to examine by adjusting the trigger event or reference to the trigger event. The disadvantages of this solution are that it is inconvenient to accomplish and may be very difficult to define a trigger event which enables the user to view the time of interest. Also, the user is prevented from any kind of simultaneous observation of primary and secondary characteristics of waveform, and the viewed waveforms will not be based on the same data, even though the waveform is repetitive.
In the second prior art solution, the oscilloscope has a second timebase circuit which is set to a separate resolution and a separate trigger event, therefore providing a separate accumulation of points which may be observed by the user on a separate display, or shared on the first display. This allows the user to observe both the primary and secondary characteristics of the waveform, and provides a higher resolution waveform for measurement purposes, consequently allowing more accurate measurements. The disadvantages of this solution are the requirement of additional hardware which requires both physical space within the oscilloscope, thereby limiting the available space for other hardware, and additional cost, which makes the oscilloscope more expensive to the user. It is typical within a digitizing oscilloscope to place the analog-to-digital converters, the trigger circuitry, timebase circuits, and acquisition memory on a single circuit board. Additional hardware and associated connections increase the possibility for unwanted noise and signal distortion. Less hardware gives the oscilloscope designer more choices in hardware placement, therefore reducing the possibility of noise on a particular board.